Typically, when a web browser requests a webpage that includes an image, when the image is downloaded it is also decoded at the time of download. Decoding an image involves expanding or decompressing the image into a form in which it can be rendered using a computer system's video hardware. Decompressing images consumes CPU resources and overhead. In addition, decompressing images can also consume memory resources.
Yet, in some situations, a referenced image may not end up being rendered and viewed. In these instances, decoding the image wastes CPU resources, overhead and memory.